Predicting Floods: The Flash Factor

Flash floods are the single deadliest storm-related weather hazard, killing more than 140 people every year in the United States. A mere six inches of moving water can knock you off your feet, while two feet of water will carry away most cars. Can we predict floods?

As the name implies, flash floods happen suddenly, leaving little time for warning (flash floods have developed in as little as 58 seconds). They usually occur on small streams with less than 20 square miles of drainage area. ("Regular" flooding may not occur until days or even weeks following a period of snowmelt or rain.)

To have any chance of predicting a flash flood, forecasters need to know where the "bullseye" of intense rain is—that is, how much rain is falling, how quickly it's coming down, and how saturated the soil is.

Radar and rain and stream gauges automatically convey this information via phone lines, radio waves, or satellites. "That way, the forecaster can receive [data] as the rain is falling," says Matthew Kelsch with the Forecast Systems Laboratory,[2] based in Boulder, Colorado. However, radar is limited in its ability to detect rain in mountainous areas. In addition, the father away you get from the radar, the less accurate the information.

Another big unknown: How will the water behave once it's on the ground? "Flash floods don't always happen in the natural stream channel," says Kelsch.

To address this, land surface characteristics such as terrain are now included in computer models along with data from radar and gauges, resulting in better predictions about how a certain location will handle a given amount of water.

For the Record

Forecast: Heavy rain, possible flooding

Fact: Thousands of campers in Colorado's narrow Big Thompson Canyon ignored warnings of flooding on July 31, 1976, with tragic results. Twelve inches of rain fell in five hours, making the river rise with astonishing speed. A 19-foot wall of water killed 139 people, as it swept away boulders, cars, bridges, even a restaurant with the diners still inside.